The present disclosure is directed to an apparatus and related method which enables measurements to be made of lengths of tubing, particularly the kind of tubing that is used in servicing an oil and gas well. It is particularly intended to be used with coil tubing which is inserted into the well at the well head from a coil of tubing supplied on a large drum or reel.
After a well has been completed, it is necessary to periodically service the well. There are many occasions where the service procedure is carried out by utilization of coil tubing. This is tubing which is relatively small in diameter, perhaps between 1 and 2 inches in diameter and which is inserted into the well head through a lubricator assembly (stuffing box) at the well head. Typically, there is a pressure differential in the well so that the well is a closed chamber producing oil or gas or a mixture from the pressurized well. The tubing that is inserted into the well is normally inserted through a lubricator mechanism which seals the well for pressure retention in the well. The tubing is inserted into the well through the lubricator mechanism by an upstanding device. The upstanding device is an injector which incorporates a set of blocks which straighten out the tubing. The tubing is flexible and can bend around a radius of curvature which enables the coiled tubing to be inserted into the well borehole through a stuffing box or lubricator. The injector mechanism is thus needed so that several thousand feet of coil tubing can be supplied on a single drum or reel and spooled off.
It has been attempted in the past to measure the length of tubing inserted into the well by positioning a measuring wheel in contact with the tubing at some point between the reel which stores the coil tubing and the injector. In this region the tubing is not straight, and it is not loaded i.e., it is not supporting weight. In other words, it is in a curved and relaxed state, and this creates difficulties for the mechanism which is required to straighten the tubing. After the tubing has been straightened by the injector, it is somewhat lengthened as it is axially loaded. Measurements of the length of tubing from any point before the tubing has been straightened and loaded provide erroneous measurements. It is important to accurately make these measurements so that the length of tubing inserted into the well can be known.
The tubing is often provided for supporting a wireline on the interior. Wireline measuring devices are well known but they cannot work where the wireline is on the inside of the tubing because the wireline measuring devices require a measuring wheel to contact the wireline under tension. It is not possible merely to adapt the wireline measuring device to the tubing because, in effect, it would measure the tubing subject to the errors mentioned namely, that the tubing is curved or coiled and also relaxed, and has not had the opportunity to be straightened.
There are additional limitations on the application of a measuring wheel to the coiled tubing. As always, the wear at the measuring wheel creates a source of error. In addition to that, there is the requirement that the wheel frictionally engage the tubing. If the loading forces that urge the measuring wheel against the tubing are insufficient, there may be slippage at that contact. Even where the loading forces are sufficient that there is good frictional engagement, there also may be another problem, namely, that the tubing will contact petroleum from the well or otherwise become externally coated to reduce friction. In any instance, the frictional loading that is applied to the measuring wheel cannot necessarily over come slippage from grease or petroleum products on the external surface of the tubing. In summary, it is just very difficult to make measurements of tubing length actually inserted into the well.
By contrast, the present system attempts to and successfully measures the length of tubing inserted into the well borehole by making connection with the chain which is involved in the injector. There is a drive chain which is aligned by a number of rollers which carries tubing engaging devices so that the tubing is grasped and straightened and is axially aligned with the stuffing box and the well head equipment. This drive chain supports a set of devices which grasp the tubing and which are known as gripping blocks. This chain is deployed with the gripping blocks in contact with the tubing so that there is no slippage. While wear of the components in this system may well occur, it does not necessarily produce slippage between chain motion and tubing motion. In other words, the travel of the chain remains precisely equal to the travel of the tubing, and especially after it has been straightened and axially loaded. The system incorporates a sprocket which is connected with the drive chain of the injector. The sprocket is rotated in proportion to the linear travel of the tubing through the injector. Moreover it utilizes a sprocket which is positively engaged with the chain so that the aggregate motion of the chain which is involved in several thousand feet of tubing provides a proportionate rotation to the sprocket. The sprocket engaging the drive chain is provided with a support shaft for its mounting, and the shaft supports an additional sprocket. A take off chain is used which is much smaller because it is not required to provide the substantial power transfer necessary for the drive chain. The take off mechanism then permits connection with additional sprockets having the required ratios thereby providing an output which is sized to movement of the chain and which carries with it a constant of proportionality so that a output pulse encoder can be driven, thereby providing necessary signals to a counter. The counter provides an output measurement which, in conjunction with appropriate pulse encoding, is enabled for direct measurement of the tubing inserted into the well. This apparatus mounts readily at the injector and is supported on the injector so that it can be permanently left there and can be dismounted with the injector for reinstallation from job to job. This accomplishes a measurement system which can be simple in operation, and which is able to operate without any impediments to its successful use.